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Taxis
A taxis (plural taxes, ) is an innate behavioural response by an organism to a directional stimulus. A taxis differs from a tropism (turning response, often growth towards or away from a stimulus) in that the organism has motility and demonstrates guided movement towards or away from the stimulus . It also differs from a kinesis, a non-directional change in activity in response to a stimulus that results in the illusion of directed motion due to different rates of activity depending on stimulus intensity. Examples For example, flagellate protozoans of the genus Euglena move towards a light source. Here the directional stimulus is light, and the orientation movement is towards the light. This reaction or behaviour is a positive one to light and specifically termed "positive phototaxis", since phototaxis is a response to a light stimulus, and the organism is moving towards the stimulus. If the organism moves away from the stimulus, then the taxis is negative. Many types of taxis have been identified and named using prefices to specify the stimulus that elicits the response. These include anemotaxis (stimulation by wind), barotaxis (pressure), chemotaxis (chemicals), galvanotaxis (electrical current), geotaxis (gravity), hydrotaxis (moisture), phototaxis (light), rheotaxis (fluid flow), thermotaxis (temperature changes) and thigmotaxis (physical contact). Depending on the type of sensory organs present, taxes can be classified as klinotaxes, where an organism continuously samples the environment to determine the direction of a stimulus, tropotaxes, where bilateral sense organs are used to determine the stimulus direction, and telotaxes, which are similar to tropotaxes but where a single organ suffices to establish the orientation movement. Aerotaxis Aerotaxis is the response of an organism to variation in oxygen concentration, and is mainly found in aerobic bacteria. Geotaxis Geotaxis is a response to the attraction due to gravity. The planktonic larvae of the king crab Lithodes aequispinus use a combination of positive phototaxis (movement towards the light) and negative geotaxis (upward movement) . Both positive and negative geotaxes are found in a variety of protozoans . Phototaxis Phototaxis is the movement of an organism in response to light. This is advantageous for phototrophic organisms as they can orientate themselves most efficiently to receive light for photosynthesis. Two types of positive phototaxis are observed in prokaryotes. Scotophototaxis is observable as the movement of a bacterium out of the area illuminated by a microscope. Entering darkness signals the cell to reverse direction and reenter the light. A second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. Chemotaxis Chemotaxis is a migratory response elicited by chemicals. Unicellular (e.g. protozoa) or multicellular (e.g. worms) organisms are targets of the substances. A concentration gradient of chemicals developed in a fluid phase guides the vectorial movement of responder cells or organisms. Inducers of locomotion towards increasing steps of concentrations are considered as chemoattractants, while chemorepellents result moving off the chemical. However, chemotaxis is described in prokaryotic and eukaryotic cells, signalling mechanisms (receptors, intracellular signaling) and effectors are significantly different. Galvanotaxis / electrotaxis Galvanotaxis or electrotaxis is directional movement of motile cells in response to a electric field. It has been suggested that by detecting and orientating themselves toward the electric fields, cells are able to direct their movement towards the damages or wounds to repair the defect. It also is suggested that such a movement may contribute to directional growth of cells and tissues during development and regeneration. This notion is based on 1) the existence of measurable electric fields that naturally occur during wound healing, development and regeneration; and 2) cells in cultures respond to applied electric fields by directional cell migration – electrotaxis / galvanotaxis. Gravitaxis Gravitaxis (known historically as geotaxis) is a response to the attraction due to gravity. The planktonic larvae of the king crab Lithodes aequispinus use a combination of positive phototaxis (movement towards the light) and negative gravitaxis (upward movement) . Both positive and negative gravitaxes are found in a variety of protozoans . Klinotaxis Klinotaxis occur in organisms with receptor cells but no paired receptor organs. The cells for reception are located all over the body, particularly towards the anterior side. The organisms detect the stimuli by turning their head sideways and compare the intensity. When the intensity of stimuli is balanced equally from all sides then the organisms move in a straight line. Examples: movement of larva of blowfly and butterfly. Magnetotaxis Magnetotaxis is the ability of certain motile, aquatic bacteria to sense a magnetic field and coordinate their movement in response. It has been suggested that by orientating themselves toward the Earth's poles, marine bacteria are able to direct their movement downwards, towards the sediment. Furthermore, bacteria that are able to metabolise metal compounds may also use magnetosomes to detect deposits of ferrous compounds. Menotaxis Menotaxis In this constant angular orientation of the organisms takes place. Example: Bees returning to their hive at night, movement of ant with response to the sun. Mnemotaxis Mnemotaxis are a complex type of stimuli. In this the organisms pick up the trails left by them when traveling back to their home. Thus this is a memory response of an organisms. Phototaxis Phototaxis is the movement of an organism in response to light: that is, the response to variation in light intensity and direction. * Negative phototaxis, or movement away from a light source, is demonstrated in some insects, such as cockroaches. * Positive phototaxis, or movement towards a light source, is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis. Many phytoflagellates, e.g. Euglena, and the chloroplasts of higher plants positively phototactic, moving towards a light source. Two types of positive phototaxis are observed in prokaryotes. # Scotophototaxis is observable as the movement of a bacterium out of the area illuminated by a microscope. Entering darkness signals the cell to reverse direction and reenter the light. # A second type of positive phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. Rheotaxis Rheotaxis is a response to a current in a fluid. Positive rheotaxis is shown by fish turning to face against the current. In a flowing stream, this behavior leads them to hold their position in a stream rather than being swept downstream. Some fish will exhibit negative rheotaxis where they will avoid currents. Telotaxis Telotaxis require paired receptors. The movement occurs along the direction where the intensity of the stimuli is stronger. For example: when bees move from their hive for food they balance the stimuli from the sun as well as flower but reside on the flower whose intensity is higher for them. Thermotaxis Thermotaxis is a migration along a gradient of temperature. Some slime molds and small nematodes can migrate along amazingly small temperature gradients of less than 0.1C/cm.Dusenbery, David B. (1992). Sensory Ecology, p.114. W.H. Freeman, New York. ISBN 0-7167-2333-6. They apparently use this behavior to move to an optimal level in soil.Dusenbery, D.B. Behavioral Ecology and Sociobiology, 22:219-223 (1988). Avoided temperature leads to the surface:… Dusenbery, D.B. Biological Cybernetics, 60:431-437 (1989). A simple animal can use a complex stimulus patter to find a location:… Tropotaxis Tropotaxis are displayed by organisms with paired receptor cells. When the stimuli coming from a source is balanced equally the organisms show movement. In this animals are capable of showing sideways movement unlike klinotaxis where the organisms show movement in a straight line. Example: movement of Greyling butterfly, fish louse See also *Animal locomotion *Automatic behavior *Instinctive behavior *Reflexes References Category:Reflexes *